The present teachings generally relate to laser based display systems.
Current projection systems used in large venues such as theaters, and museums, based on lamp-based projection sources, cannot provide the high contrast ratio required for these specialized applications. These projectors typically combine multiple high power lamps (250-500 W) in order to provide sufficient light projection. Xenon lamps of 6000-7000 W have also been employed in digital cinema projectors. However, the large etendue (the spread of the light in area and angle) of the lamps makes for inefficient light collection and projection. Moreover, the cost of ownership for such equipment is considerable due to the high electrical consumption associated with operating and cooling the projectors and the high cost of lamp replacements because high power Xenon lamps (i.e., >3,000) have very short lifetimes (e.g., 750 hours) and relative high cost $4,900 (e.g., www.ProjectorLampExperts.com). Part of the generated emission is wasted as the lamps emit over a very wide spectrum of wavelengths (e.g., from W to IR), and optical filters are required to allow only the three primary colors to be transmitted through the system. The brightest xenon lamp projector in the market uses 20,000 ANSI lumens. The average purchasing cost is $120,000, without the necessary projection lenses. The drawbacks of such a system is the low contrast ratio of 1,800:1, the very short lifetime (e.g., 750 h) and high lamp replacement cost. It is also important to note that lamp based projectors color quality quickly degrades, even if the lifetime of the lamp has not reached its life time limit.
Since the 80's, it was suggested that laser projection systems offer many benefits over other projection technology. Laser light is far more directional, powerful, and coherent than any other light source. Hence, lasers produce an intense beam that is very pure in color. Pure colors produced by red, green, and blue lasers are the primary wavelengths on the boundary of the ICI (CIE) chromaticity diagram, a characteristic that allows a very large gamut of colors (FIG. 1) compared to CRT and TFT LCD technologies. This maximizes the color space available for displaying highly saturated colors. Laser approaches also offer the advantage of stable laser color (wavelength) over the lifetime of the laser, which can reach more than 15,000 hours. In addition, unlike deformable oil films, phosphors, and LCDs, a laser has zero persistence. Moving images in a laser display will not smear or blur, hence high contrast ratios can be achieved (e.g., >15,000:1) Due to the excellent laser beam collimation characteristics, which results in greater depth of focus, it is possible to permit projection on curved surfaces with high resolution. Hence, laser projection promises an increased color gamut, higher luminance, zero persistence, and increased line rates.
Today's typical laser based projection configuration is 30 shown in FIG. 2. The system consists of:
(a) Sources (typically one or two, polarization multiplexed lasers) for each color
(b) Modulation scheme
(c) Optical path delivery
(d) Projection head
To date the major research and development effort in improving the system performance has been in the development of new and more powerful laser sources as well as the development of better, more efficient and better contrast ratio modulation schemes. Nevertheless, the major hurdle in the widespread use of laser in video projection systems has been the high cost of the lasers. There have been two main directions to fulfill the need for high optical power lasers to serve as the light source of these systems. These are:
(a) The development of individual diode pumped solid-state lasers (DPSSL), and improved non-linear optical frequency conversion techniques to build efficient and reliable high power visible laser sources. These approaches have a physical/engineering limit to the maximum optical power they can achieve, and in particular of the blue laser.
(b) The development of systems that generate all three primary colors from one laser source. These are based either on disk laser or optical parametric oscillator (OPO) and several non-linear stages to generate high optical powers (e.g., total power 18 W). Nevertheless, these approaches lead to expensive and large laser systems (e.g., 2 m×1.5 m×1 m) that require massive heat dissipation, high electrical power and are vulnerable to laser failures. That is if the laser malfunctions the whole system needs to be repaired and/or replaced.
Due to the different response of the eye to different wavelengths, a balanced white light is difficult to be accomplished. Higher optical powers are required for the red and blue wavelengths compared to the green laser. Hence, often wastage of the green optical power (which is easier to achieve) is required to provide the balanced white light. At the same time, due to the different response of the eye in bright or dark environment the optical power requirements change significantly depending on whether the observer is in a dark or bright room. Hence, there is a need for a system that can offer high contrast and saturation images for simulation and training as well as planetarium applications in both dark and bright environments.